Chlorine dioxide generation for water treatment

ABSTRACT

Chlorine dioxide is generated by electrochemical oxidation of sodium chlorite in an anode compartment of a cation-exchange membrane-divided cell and is recovered in a suitable recipient medium by passing the chlorine dioxide through a hydrophobic microporous membrane. Water balance in a continuous operation is maintained by removing water from the anolyte by transporting the same partly across the hydrophobic microporous membrane in vapor form and partly across the cation-exchange membrane.

This application is a 371 of PCT/CA94/00263 filed May 12, 1994.

FIELD OF INVENTION

The present invention relates to an enviromentally-friendly, continuousprocess for the production of chlorine dioxide for water treatmentapplications in a very pure form, substantially free from contaminants,such as chlorine, chlorite, chlorate and chloride.

BACKGROUND TO THE INVENTION

In U.S. Pat. No. 4,683,039, assigned to the applicant and the disclosureof which is incorporated herein by reference, there is described theproduction of chlorine dioxide and its separation using gas membranes,i.e. microporous hydrophobic membranes which permit gas or vapor ofchlorine dioxide to pass therethrough but resist the passage of liquidand ions therethrough.

U.S. Pat. No. 2,163,793 describes an electrochemical chlorine dioxidegenerating process in which a mixture of alkali metal chlorite andalkali metal chloride is electrolyzed in an electrolytic cell equippedwith a porous diaphragm separating the anode and the cathodecompartments.

British Patent No. 714,828 describes a process for the production ofchlorine dioxide by electrolysing an aqueous solution containingchlorite and a water soluble salt of an inorganic oxy-acid other thansulfuric acid while U.S. Pat. No. 2,717,237 discloses a method forproducing chlorine dioxide by electrolysis of chlorite in the presenceof sulfate ions.

Japanese Patent Publication No. 81-158883, published Dec. 7, 1981,describes an electrolytic process for producing chlorine dioxide byelectrolysis of chlorite in which the electrolysed solution, at a pH of2 or less, is fed to a stripping tank where air is introduced to recoverthe chlorine dioxide.

U.S. Pat. No. 4,542,008 describes an electrolytic process for chlorinedioxide production in which the sodium chlorite concentration in theanolyte is monitored and controlled by means of a photometric cell.

Published International patent application WO 91/09158 discloses amethod of producing chlorine dioxide from chlorite in an ion exchangecompartment of a multi-compartment cell in which hydrogen ions generatedin the anode compartment enter the ion exchange compartment through acation exchange membrane, causing chlorite ions decomposition to formchlorine dioxide and other by-products.

Published International patent application WO 91/09990 teaches anelectrochemical process for producing chlorine dioxide from dilutealkali metal chlorite solution in a single pass mode using a porousflow-through anode in which the unconverted chlorite together with othercomponents of the anolyte constitute an effluent.

A disadvantage of all of the above described electrolytic processes forthe production of chlorine dioxide is that they are not suitable for ahighly efficient, continuous, effluent-free operation in which all thecomponents of the chlorite feed are safely and very efficiently removedwith formation of an essentially pure chlorine dioxide, whereby noundesired or harmful by-products or contaminants are generated oraccumulated.

SUMMARY OF INVENTION

In the present invention, there is employed a combination of highlyefficient electrochemical oxidation of sodium chlorite to chlorinedioxide and membrane separation of the chlorine dioxide so produced incontinuous production of chlorine dioxide. Accordingly, in oneembodiment of the present invention, there is provided a method for theproduction of chlorine dioxide, which comprises electrochemicallygenerating chlorine dioxide from an aqueous solution of an alkali metalchlorite, particularly sodium chlorite, and recovering the chlorinedioxide so produced by passing the chlorine dioxide through ahydrophobic, microporous membrane to a recipient medium.

However, a broad aspect of the present invention provides a method ofremoving at least one dissolved gas and water from an aqueous solutionof the at least one gas, which comprises contacting the aqueous solutionwith one face of hydrophobic microporous membrane; and providing adifferential of partial pressure of both the at least one gas and watervapor between the aqueous solution and a recipient medium in contactwith the opposite face of the hydrophobic microporous membrane, wherebyboth the at least one gas in gaseous form and water vapor pass throughthe membrane.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a schematic flow sheet of a chlorine dioxide generator andutilization process provided in accordance with one embodiment of theinvention.

GENERAL DESCRIPTION OF INVENTION

Electrochemical production of chlorine dioxide is effected in anelectrochemical cell, divided by an ion-permeable separator, generally acation-exchange membrane, into an anode compartment and a cathodecompartment. The electrochemical reaction to form chlorine dioxide iseffected in the anode compartment of the cell from a feed thereto of anaqueous solution of alkali metal chlorite, usually sodium chlorite. Asuitable aqueous electrolyte is provided to the cathode compartment.Chlorine dioxide and water vapor are transferred to the recipient mediumthrough a hydrophobic microporous membrane.

The production of chlorine dioxide from sodium chlorite byelectrochemical oxidation in the anode compartment in accordance withthe equation:

    NaClO.sub.2 →ClO.sub.2 +e+Na.sup.+                  (1)

enables highly efficient production of chlorine dioxide to be achieved,essentially uncontaminated with chlorine. The use of a gas membraneprocess (or pervaporation as it is termed in the above-noted U.S.Patent) enables highly pure chlorine dioxide to be recovered from thecell liquor and transferred to a recipient medium, as desired.

An efficient chlorite oxidation process preferably is carried out atapproximately neutral pH. Since the typical technical product which canbe used as a feed to the chlorine dioxide generation system, forexample, about 37 wt % sodium chlorite solution, usually contains somesodium hydroxide and/or carbonate/bicarbonate as a stabilizer, it isbeneficial to adjust the pH of the sodium chlorite feed accordingly.

Another source of hydroxide ions that can be present in the anolyte isthe so-called "backmigration" of hydroxyl ions originating in thecathode compartment of the electrochemical cell employed to effectreaction (1) and entering the anolyte through the cation exchangemembrane or another ion-permeable separator. While a cation exchangemembrane is, in principle, permeable only to cations, its anionrejection characteristics is usually not perfect, thus allowing alimited quantities of anions, such as hydroxyl ions to penetrate to theanode compartment. Some cation exchange materials are known to have abetter anion rejection characteristics than others but in general acertain degree of hydroxyl ions backmigration can be anticipated. It isknown, for example, that cation exchange membranes based on thecomposite of perfluorinated carboxylic and sulfonic cation exchangecopolymers have a better selectively than the polymers equipped withsulfonic groups only. The former membranes, however, are much moresensitive to the presence of impurities, such as hardness forming ions,than the latter membranes.

The anion rejection characteristics of the cation exchange material isimportant, not only from the viewpoint of the minimization of thehydroxyl ion backmigration to the anolyte but also with regard topotential contamination of the catholyte by the anionic species presentin the anolyte, such as chlorite ions. The backmigration of hydroxylions to the anolyte may also be influenced by the concentration of suchions in the catholyte.

The presence of excess alkali in the anolyte may lead to a well-knownchlorine dioxide disproportionation reaction and should be avoided. A pHadjustment of the anolyte may be made by any convenient method,including an electrochemical method.

Such electrochemical method is based on the occurrence of the oxygenevolution reaction at the anode:

    2H.sub.2 O→O.sub.2 +4H.sup.+ +4e                    (2)

in addition to the primary electrode reaction depicted by equation (1).

Since reaction (1) is thermodynamically more favourable than reaction(2), one possible method of enabling the occurrence of reaction (2) isto impose an anodic current density which cannot be fully sustained byreaction (1), i.e. by operating the process at a current densityexceeding that corresponding to a mass transport limitation of reaction(1). The mass transport limitation of electrode reaction (1) is believedto be a function of such factors as chlorite ion concentration, anolytecomposition, flow rate (velocity), presence of turbulence, temperatureand anolyte viscosity.

Relative contributions of reactions (1) and (2) to the overall currentcan also be modified, for example, by the pH of the anolyte and by theselection of the anode material. It is known that certain oxygenevolving anodes, such as DSA-O₂ ® anodes (trademark of ELTECH CORP.),are characterized by a lower oxygen evolution overpotential than otheranode materials, such as, for example, graphite, thus facilitating theoccurrence of reaction (2). The oxygen evolution characteristic of theanode is one of many factors to be considered for proper selection ofthe anode material. Other factors are, for example, the anode effects onthe decomposition of chlorite ions and chlorine dioxide. Anode materialfacilitating such decomposition or oxidation of chlorine dioxide tochlorate ions should be avoided. When using a plurality ofelectrochemical cells it is possible to combine cells equipped withanodes made from different electrode materials. For example, some cellsmay be equipped with anodes promoting reaction (2), such as DSA-O₂ ®anodes, while other with anodes enabling a highly efficient (i.e. withno undesired by-product formation) reaction (1), such as graphite,which, on the other hand, may not be sufficiently stable underconditions of oxygen evolution reaction (2). The current density imposedmay differ for different anode materials serving different objectives inthe multi-cell assembly. For example, if oxygen evolution is to beavoided on graphite anodes, the current density on such anodes shouldnot exceed that governed by mass transport limitations. The costs of theanode material should also be considered during optimization.

There are numerous anode materials in various shapes and forms which maybe considered for the proper balancing of reactions (1) and (2). Typicalanode materials include not only above mentioned graphite (or any othercarbon material, such as glassy carbon) and DSA-O₂ ® but also leaddioxide, platinum or other noble metal (both taken alone as well asconnected to a suitable substrate), ruthenium dioxide on titanium (knowne.g. as DSA-Cl₂ ®--Trademark of ELTECH CORP.), platinum/iridium ontitanium, for example. The effective surface of the anodes may beenhanced, if required, by utilizing their three-dimensional structures,the latter being particularly useful for generation of chlorine dioxidefrom dilute anolyte streams. A method of preparing such structures hasrecently been described in U.S. Pat. No. 5,298,280.

The extend to which reaction (2) is permitted to occur may be calculatedbased on the overall input of hydroxyl ions to the anolyte. Thus, forexample, a more pronounced hydroxyl ions backmigration from thecatholyte (resulting, for example, from a higher concentration ofhydroxyl ions in the catholyte) would usually require, but not always, ahigher anodic current density in order to increase the contribution ofreaction (2).

It is possible to employ a pH measurement in the anolyte as a feedbackfor the adjustment of current density. At a given anolyte composition(in terms of chlorite concentration) and for a given flow conditions, anupward trend of pH would trigger, in such case, an increased currentdensity by either increasing the overall current at a given overallsurface area of the anodes or, if an approximately constant productionrate of chlorine dioxide is desired, the current may be maintained at aconstant level while the overall surface area of the anodes is decreased(by, for example, switching off anolyte flow to the part ofelectrochemical cells comprising a multi-cell assembly). Alternatively,a feedback originating from the pH measurement of the anolyte may alsolead to an automatic or manual adjustment of the anolyte composition(chlorite concentration), flow conditions, temperature etc. Similarly, adownward trend in pH may automatically trigger the required adjustmentsin current density, chlorite concentration, flow conditions, temperatureetc.

It is believed that lasting excursions of the anolyte pH to the acidicrange may result in a decomposition of chlorite ions leading to theformation, in addition to chlorine dioxide, of at least one of thefollowing impurities, namely chloride ions, chlorate ions and chlorine.Chlorine may originate, for example, from a secondary reaction involvingelectrooxidation of chloride ions at the anode. Such reaction may not beentirely undesired since chlorine is known to react rapidly withchlorite ions to form chlorine dioxide and chloride ions. Chlorineevolution characteristics of the anode is yet another factor to beconsidered for the proper anode selection. Such excursions, therefore,should generally be avoided.

It is further believed that small, temporary excursions from the optimumpH of about 7 of the anolyte to either alkaline or acidic range aregenerally acceptable. The impact of such excursions is a function ofsuch factors as chlorite concentration, chlorine dioxide concentration,temperature, time, etc. For example, the higher the chlorine dioxideconcentration, the more pronounced would be its decomposition during pHexcursions to the alkaline pH range. The excursions to the slightlyacidic pH range are believed to be generally more acceptable than thoseto the alkaline pH range. It has been reported in the literature thatthe occurrence of the undesired electrooxidation reaction of chlorinedioxide to chlorate can be minimized by maintaining the pH of theanolyte in the slightly acidic range.

Since hydroxyl ions originating from the catholyte and entering theanolyte through the cation exchange membrane as a result of thebackmigration may create a strongly alkaline boundary layer on thesurface of the membrane facing the anode and since the compensating(neutralization) reaction producing hydrogen ions (reaction 2) takesplace at the anode, which may create a strongly acidic boundary layer inthe proximity of the anode which is located at a certain, finitedistance from the alkaline boundary layer, it may be beneficial tocreate conditions facilitating the neutralization reaction between thehydroxyl and hydrogen ions in order to minimize the occurrence ofpossible undesired reactions involving, for example, chlorite ions in alocalized acidic medium or chlorine dioxide in a localized alkalinemedium. It is possible, for example, to improve the mixing within theanode compartment by improving the flow characteristics and employingturbulence promoters. Any suitable pH buffer, such as phosphate buffer,phthallate buffer, citrate buffer or a combination thereof, may beemployed in order to moderate the pH changes within the anodiccompartment. Such buffer should be characterized by a sufficientbuffering capacity in terms of its ability to "absorb" hydroxyl ions andhydrogen ions without introducing any kinetic limitations on theneutralization process. More concentrated buffers are generally known tohave higher buffering capacity. The effect of an alkaline boundary layerin the proximity of the surface of the cation exchange membrane facingthe anode can also be minimized by employing a slightly acidic anolyte,if desired.

Accordingly, in a further aspect of the invention, there is provided anelectrochemical process in an anode compartment of an electrochemicalcell divided by at least one ion-permeable separator from a cathodecompartment, which comprises feeding a first electrolyte to the anodecompartment and effecting electrolysis of the electrolyte to formhydrogen ions; feeding a second electrolyte to the cathode compartmentand effecting electrolysis of the electrolyte to form hydroxyl ions, atleast some of the hydroxyl ions backmigrating across the at least oneion-permeable separator into the anode compartment; and providing abuffer in the anode compartment to neutralize both theelectrochemically-produced hydrogen ions and the back-migrating hydroxylions as well as the hydroxyl ions which may be introduced to the anodecompartment with the first electrolyte feed.

While reaction (2) is a preferred reaction generating the necessaryhydrogen ions required for the neutralization of hydroxyl ions enteringthe anolyte loop, some other electrochemical reactions may also beemployed. An example of an electrochemical reaction resulting in thegeneration of hydrogen ions without a formation of any persistentbyproducts is the electrooxidation of hydrogen peroxide.

    H.sub.2 O.sub.2 →2H.sup.+ +2e+O.sub.2               (3)

Alternatively, metal peroxides or superoxides, preferably sodiumperoxide, may be employed to substitute hydrogen peroxide in reaction(3).

There are many other reagents, in particular organic compounds, whichmay generate hydrogen ions during oxidation without co-production of anypersistent by-products. Preferably, simple organic compounds such asmethanol or other simple alcohols, aldehydes, ketones, acids or theircombination can be employed whereby the electrooxidation reactionresults in the formation of hydrogen ions and carbon dioxide. An exampleof such reaction is depicted by equation (4):

    organic compound+water→pCO.sub.2 +nH.sup.+ +me.sup.-(4)

where the coefficients p, n and m are based on the stoichiometry ofreaction (4).

Carbon dioxide generated in reaction (4), if not stripped from thereaction medium, may form bicarbonate or carbonate ions which couldcomprise a persistent impurity. Nitrogen based reagents such as ammonia,urea, hydrazine and hydroxylamine may also be employed, whereby theelectrooxidation reaction preferably produces nitrogen gas and hydrogenions.

While it is preferred to operate the process of the inventioncontinuously without generating any undesired effluent or by-productsand thus it is beneficial to employ a hydroxide neutralization methodwhich fulfills such requirements, it is still possible to utilize aconventional acid or acid anhydride (such as sulfuric or phosphoric acidor carbon dioxide, the latter being equivalent to carbonic acid)addition, especially if the build-up of persistent contaminants, such assulfate, phosphate, bicarbonate or carbonate (or other ions, if adifferent acid or acid anhydride is employed), is counteracted byperiodic removal of impurities by any convenient method, such asbleed-out, precipitation, chemical decomposition and membraneseparation.

Such approach may be particularly acceptable when a non-continuous,batch-wise operation is employed. Any of the above mentioned methodsalso can be employed to counteract the build-up of impurities in acontinuous operation involving a preferred, electrochemical method ofhydroxyl ions neutralization. Even during such operation some of theundesired contaminants, such as chlorate ions, may be formed or may beadded with the chlorite feed and hence a suitable method for the anolytepurification may be incorporated into the process, if necessary.

Since the typical, commercially available sodium chlorite usually maycontain, in addition to the previously mentioned hydroxide, smallquantities of various contaminates, such as carbonate, chlorate, sulfateand chloride, which may accumulate to unacceptable levels during aprolong, continuous operation, it is beneficial to modify the sodiumchlorite manufacturing process in order to produce a sufficiently puresubstrate intended for use as a feed to the chlorine dioxide generationprocess. Alternatively, the commercially available sodium chlorite maybe subjected to a suitable purification prior to being use as a feed.

While various above-described methods for pH adjustment, specificallythe electrochemical method depicted by reactions (2) to (4), have beendisclosed in the context of the electrooxidation of chlorite ions tochlorine dioxide, their applicability is not limited to this processonly and can be extended to any electrochemical process in which theproper balancing of pH is required.

Accordingly, in another aspect of the invention, there is provided anelectrochemical process in an anode compartment of an electrochemicalcell divided by at least one ion-permeable separator from a cathodecompartment to which an aqueous electrolyte is fed, which compriseseffecting electrochemical oxidation of an aqueous salt solution in theanode compartment while transferring the cation species of the salt tothe cathode compartment; and simultaneously effecting electrochemicalacidification of the aqueous salt solution to effect pH constancy of theaqueous salt solution.

An analogous approach can also be applied to electroreduction reactionswhen the primary electrochemical reaction occurs at the cathode and whenthe pH balancing can be achieved by, for example, electrogeneration ofhydroxyl ions at the cathode to any desired degree. Again, no persistentimpurities are accumulated during such operation which is of greatimportance for continuous environmentally-friendly processes.

A major problem associated with operating the chlorine dioxidegeneration process according to the invention and depicted by equation(1) on a continuous basis is maintaining a proper water balance. Forexample, using a concentrated (37 wt %) sodium chlorite solution as afeed introduces water to the electrochemical cell at a mole ratio ofNaClO₂ :H₂ O of about 1:9. Under conditions of a 100% efficient sodiumchlorite electrooxidation process effected in a two-compartmentcompartment cell equipped with a cation-exchange membrane to producechlorine dioxide, approximately 2 to 5 moles of water is transportedthrough the membrane to the cathode compartment with each mole of Na⁺ions, which corresponds to 2 to 5 moles of water removed from theanolyte for each mole of ClO₂ produced. However, to maintain the properwater balance required for a continuous operation of the chlorinedioxide generation process, an additional 4 to 7 moles of water per moleof chlorine dioxide produced must be removed from the anolyte. Theamount of water to be removed from the anolyte may be higher if a lessconcentrated feed of sodium chlorite is used. The amount of water alsomay be lower, for example, if crystalline sodium chlorite is employedand the amount of water fed to the system is smaller.

In accordance with the present invention, the further volume of waterrequired to be removed is removed by a gas membrane process, similar tothat used for separation of chlorine dioxide from cell liquor. While thehydrophobic microporous membrane used in this process does not allowliquid water transport, it has been found that, it is possible totransfer water through this membrane in a vapor form.

Accordingly, the chlorine dioxide-containing anolyte from theelectrochemical oxidation of sodium chlorite is maintained at a positivewater vapor pressure gradient with respect to the chlorine dioxidereceiving solution (i.e. the water treated by the chlorine dioxidetransferred from the anolyte) by employing, for example, a highertemperature of the chlorine dioxide donating solution, in order toeffect transfer of water vapor through the gas membrane along with thechlorine dioxide.

The temperature differential between the donor and recipient medium tofacilitate the water vapor passage may vary from about 0.1° to about100° C., preferably about 1° to about 50° C., to facilitate the transferof water vapor. Accordingly, the donor medium may have a temperaturefrom about 5 to about 100° C., preferably about 15 to about 80° C. whilethe recipient medium may have a temperature of about 1 to about 80° C.,preferably about 5 to about 40° C. A pressure differential may be usedalternatively to or in conjunction with the temperature differential toprovide the vapor transfer driving force.

The composition of the chlorine dioxide donating solution may affect thewater vapor gradient thus influencing the effectiveness and the rate ofwater transfer. Other important factors to consider in this respect isthe flow characteristics of both the donating and receiving solutionsand their temperatures. The hardware design of the gas transfer moduleequipped with the gas membrane plays a very important role in ensuring aproper flow characteristics by allowing a desired flow rate (velocity)and turbulence.

The removal of water from the chlorine dioxide generator, therefore, iseffected by a combination of two membranes, one a cation-exchangemembrane in the electrolysis step and the other a gas membrane, tomaintain continuous chlorine dioxide production for water treatment.While it is usually beneficial to employ a common gas membrane for bothchlorine dioxide and water vapor transfer, it is also possible toutilize separate gas transfer modules for each of the processes, wherebymembrane characteristics may be optimized according to the requirementsof each process. Alternative water removal procedures may be adopted, ifdesired, such as membrane distillation, reverse osmosis, andevaporation. A combination of various methods may also be employed.

It is further believed that analogously to the case of water vaportransfer where the water vapor pressure gradient is considered to be themain driving force for such process, the transfer of chlorine dioxidebetween the donating and receiving solutions is also governed by asimilar driving force, i.e. the chlorine dioxide vapor pressuredifferential. Similarly, as in the case of water vapor transfer, severalfactors should be considered during the optimization of the chlorinedioxide transfer. The chlorine dioxide vapor pressure and gradient (andresulting flux and transfer rate) can be manipulated by adjustingchlorine dioxide concentration, chlorite concentration and flowcharacteristics (flow rate, velocity and turbulence) of both donatingand receiving solutions as well as the temperatures of both thesolutions. Addition of any suitable salt, such as sodium chloride orsulfate as well as pH buffers may also affect the vapor pressure of bothchlorine dioxide and water.

The gas membrane material used in the gaseous transfer steps may affectthe effectiveness and the rate of the gas transfer. The importantfeatures to be considered in this respect are the hydrophobicity of themicroporous material, pore size, thickness, chemical stability towardsthe attack of chlorine dioxide, chlorine, chlorite, chlorate, chloride,acid and base, a so-called "bubble point" (related to a maximum pressuredifferential between the donating and receiving solutions that can beattained before the contact between both solutions is effected, leadingto highly undesired contamination of the receiving solution by the ionspresent in the donating solution).

Examples of such materials and their characteristics have been describedin the aforementioned U.S. Pat. No. 4,683,039. One material specificallyrecommended in the above mentioned U.S. Patent is expandedpolytetrafluoroethylene, which is commercially-available under thetrademark "GORE-TEX". Another material which exhibits a satisfactoryperformance is known as polyvinylidenefluoride (PVDF). Other materialsmay also be employed. However, at this time, the polytetrafluoroethylenematerial appears to have a superior performance, especially in terms ofits chemical resistance towards the attack of strong oxidizers, such aschlorine dioxide, chlorine and chlorite ions.

The hydrophobic microporous materials can be manufactured in variousforms, such as sheets, hollow fibers, tubes and spirals and sealed inthe appropriate modules. It is possible to design a module in which ananodic compartment in an electrochemical cell is adjacent to the gasmembrane so that the donor compartment of the gas membrane unit and theanode compartment of the electrochemical cell are combined into one,common chamber.

If desired, the content of the cathodic compartment of the electrolyticchlorine dioxide production, comprising mainly aqueous alkali, may beadded to the recipient medium, before or after chlorine dioxidetreatment. The relatively small volume of catholyte effluent compared tovolume of water treated should result in an insignificant change in pHin the treated water. Since the produced hydroxide has many applicationsin the water treatment facilities, the production of hydroxyl ions by anelectroreduction of water is a preferred cathodic reaction. However, anyother suitable cathodic reaction may be employed, for example, theelectroreduction of oxygen, resulting in a mixture of perhydroxyl andhydroxyl ions and at the same time allowing a lower cell voltage.Perhydroxyl produced in this reaction may be used as a disinfectingagent in water treatment (separately or together with chlorine dioxide)or other suitable application.

The catholyte may comprise also an acidic electrolyte, such as sulfuricacid, sulfurous acid, phosphoric acid, carbonic acid, hydrochloric acidor any other acid, the corresponding salts or their mixtures with acidsand/or acid anhydrides. The selection of the cathode material depends onthe nature of the catholyte. In general, the cathode materials suitablefor alkaline medium, such as, mild steel or nickel, may not besufficiently resistant against the corrosive attack of acidicelectrolytes. In the latter case, the cathode materials such asgraphite, lead, lead dioxide, Ebonex®, titanium, tantalum, zirconium,various metalloids, such as metal carbides or nitrides, as well asvarious noble metals or noble metals coated electrodes, may be employed.

Various pH buffers, such as phosphate, citrate, borate, phthallate,carbonate, acetate, ammonium or other buffers, may also be employed ascatholyte solutions. Maintaining the pH of the catholyte below thestrongly alkaline range corresponding to the presence of free hydroxidemay be beneficial having regard to the minimization of the previouslymentioned backmigration of the hydroxyl ions to the anolyte.

Since in most cases the primary reaction occurring at the cathodegenerates hydroxyl ions via decomposition of water or corresponds to thecathodic discharge of hydrogen ions present in the acidic catholyte, thepH of the catholyte may increase in the course of electrolysis. In suchcase, it may be beneficial to maintain the pH of the catholyteapproximately constant by employing any suitable method, such as acid oracid anhydride (e.g. carbon dioxide) addition, by dilution, by chemicaldecomposition or by precipitation. The content of the catholyte loop maybe continuously or periodically discharged in any suitable manner,either to the sewage or to the treated water. For example, by employinga catholyte containing bicarbonate, carbonate or their mixtures, it ispossible to co-produce a very useful by-product, such as soda ash, whichis readily applicable in the water treatment facilities, while, at thesame time, maintaining the catholyte pH below the strongly alkalinerange, so that the concentration of free hydroxyl ions and hence theirback-migration to the anolyte is minimized. The rejection of bothbicarbonate and carbonate ions by the cation exchange membrane isusually much more efficient than that of hydroxyl ions. The pH of thebicarbonate/carbonate mixture can easily be adjusted by eithercontinuous or periodic addition of carbon dioxide or other acid/acidanhydride. It is also possible to employ a continuous, single-pass (orwith recirculation) cathodic process in which bicarbonate orbicarbonate/carbonate mixture is fed to the cathode compartment and theproduct enriched in carbonate is continuously withdrawn from thecompartment.

While using an acidic catholyte, it is possible to utilize the effect ofhydrogen ions "leak" to the anolyte through the cation exchangemembrane. The extent of the hydrogen ion "leak" may be adjusted to matchthe quantity of hydroxyl ions entering the anolyte loop with the sodiumchlorite feed.

The cathodic reaction may result in the co-production of other usefulby-products. For example, a catholyte containing bisulfite or sulfiteions or their mixtures with sulfur dioxide may be utilized for theco-production of sodium dithionite, which is a known bleaching agent.

It is also possible to utilize a special cell design in which thedistance between the cathode and the cation exchange membrane isminimized (a so-called "zero gap" approach), so that a very diluteelectrolyte, even water containing very small quantities of ions,resulting for example, from the dissociation of the carbonic acid beingin equilibrium with the naturally present, dissolved carbon dioxide, maybe employed without requiring an unacceptably high cell voltage. Smalladditions of any additive which produces ions in water, i.e. acids,bases, salts, acid anhydrides, oxides etc., may be employed, if desired,in order to improve the conductivity of the catholyte.

A so-called "single pass" continuous operation may be employed, wherebythe catholyte is not recycled, thus allowing for simplification of theequipment required. It is possible, for example, that the treated waterbe subjected to cathodic reaction in a once-through system without beingrecycled.

Similarly to the "zero gap" approach, a so-called solid polymerelectrolyte (SPE) configuration also may be employed, whereby thecathode and/or the anode or both remains in an intimate contact with thecation exchange membrane.

Alternatively to an electrolytic cell equipped cation exchange membrane,a yet another approach can be taken in which the catholyte and theanolyte compartments are separated by means of a conducting ceramicmaterial which preferably allows a specific ionic transfer of sodiumions. Numerous examples of such materials are described in the priorart, e.g. beta-alumina or various ceramic materials described in U.S.Pat. No. 5,290,405. Selectivity of the ceramic material towards thesodium ion transfer is required when sodium chlorite is employed as asource of chlorite ions in the anolyte. However, the utilization ofother chlorites, preferably alkali metal or alkali earth metalchlorites, is also possible and, in such a case, the selection of anappropriate ceramic separator allowing an ionic transfer of a desiredmetal ion may be required.

While a two-compartment electrolytic cell equipped with one separatorbetween the anode and the cathode compartment is preferred for effectingthe electrooxidation of chlorite ions to chlorine dioxide, amulticompartment cell containing at least one additional compartmentbetween the cathode and anode compartments also may be employed. Acentre compartment so-provided is preferably separated from the adjacentcathode compartment by means of a cation exchange membrane.

The separation of the centre compartment from an adjacent anodecompartment may preferably be effected by means of either acation-exchange membrane or an anion-exchange membrane. In the lattercase, the sodium chlorite feed should be directed to the centrecompartment from which the chlorite ions and sodium ions resulting fromthe dissociation of the sodium chlorite are transferred through theanion exchange membrane to the anode compartment and through the cationexchange membrane to the cathode compartment, respectively. Theanion-exchange membrane selected for such three-compartment cell shouldpreferably be resistant to the attack of strong oxidizers, such aschlorine dioxide, chlorine and chlorite.

When the three-compartment cell is equipped with two cation exchangemembranes the sodium chlorite feed should be directed to the anodecompartment, while the centre compartment can be utilized as a bufferingcompartment preventing penetration of the anode compartment by hydroxylions generated in the cathode compartment. Elimination of the hydroxylion back-migration to the anode compartment by employing amulti-compartment cell allows to increase the contribution of reaction(1) to the overall current, but at the same time, it is associated withan increased cell voltage and higher cost of the equipment required.

The optimization of the overall process comprising two basic operations,i.e. chlorine dioxide electrogeneration and chlorine dioxide and watervapor transfer through the gas membrane requires careful balancing ofprocess parameters which may differently affect each of above mentionedoperations.

For example, the steady state concentration of chlorine dioxide in theanolyte/donating solution, while having positive effect on the rate ofgas transfer through the gas membrane, may accelerate chlorine dioxidedecomposition reactions to undesired by-products. Chlorine dioxideconcentrations may typically be varied between 0.01 gram per liter and20 grams per liter, preferably 1 to 10 grams per liter. The chlorinedioxide concentration in the receiving solution will typically be lowerthan in the donating solution.

Chlorite ion concentration in the anolyte/donating solution may affectboth the electrochemical cell performance and the gas membrane moduleoperation in several ways. In general, higher chlorite ion concentrationresults in an increased partial pressure of chlorine dioxide whiledecreasing the partial pressure of water vapor, thus enhances thetransfer rate of chlorine dioxide while decreasing the transfer rate ofwater vapor. Increased chlorite ion concentration also improves theconductivity of the anolyte thus resulting in a reduction of the cellvoltage. This latter effect, i.e. an increase of conductivity may alsobe achieved by an addition of any suitable electrolyte to the anodecompartment. Such electrolyte may be either electrochemically inert,such as, sodium sulfate, nitrate, carbonate, phosphate, perchlorate,etc, or electroactive, such as sodium chloride. Since the mass transportlimited current corresponding to reaction (1) is proportional tochlorite ion concentration, higher current densities are generallyeasier to attain for more concentrated anolytes. On the other hand,however, the decomposition rate of chlorite ions is generally enhancedby increasing chlorite ion concentration.

It is believed that the decomposition of chlorite ion is usuallypreceded by its protonation in which an unstable chlorous acidintermediate is formed. It is further believed that the source ofchlorous acid may be the well-known hydrolysis reaction of chloriteshown in the following equation:

    ClO.sub.2.sup.- +H.sub.2 O⃡HClO.sub.2 +OH.sup.-(5)

The pH of resulting solution can be approximated by the followingequation:

    pH=7+1/2 pKa+1/2 log C                                     (6)

where Ka denotes the dissociation constant of chlorous acid (pKa approx.2) and C denotes the molar concentration of chlorite ions. Using formula(6), it can be shown that, at even moderately concentrated (>1 molar)chlorite ion solution, the "natural" pH is higher than 8. An adjustmentof pH to a lower value, such as pH=7, would trigger an approximately tenfold increase in the equilibrium concentration of chlorous acid, which,in turn, may result in an increase of the undesired decomposition ofchlorite in which at least one of chlorate ions and chloride ions areformed along with some chlorine dioxide and chlorine. The higher thechlorite ion concentration the more decomposition can be expected uponthe adjustment of pH to approximately neutral. The concentration ofchlorite ions may also affect the extent of the formation of a complexcompound between chlorite ion and chlorine dioxide.

The "natural" pH value for solutions containing a lower chlorite ionconcentration is closer to 7 and hence less decomposition can beanticipated under such conditions. In general, the steady stateconcentration of chlorite ions should be optimized based on the properbalance of all the above described effects while maintaining the rate ofthe chlorine dioxide formation corresponding the chlorite oxidationcurrent density at a commercially-acceptable level. The current densitystandardized on the membrane area is usually in the range between about0.5 and about 10.0 kA/m², preferably between about 1 and about 4 kA/m².Lower concentrations of chlorite ions do not necessarily translate intolower current density attainable, since there are other variablesavailable to manipulate its value. For example, high surface area anodesmay be employed for processing of dilute chlorite ion solutions, wherebythe conductivity of the anolyte may optionally be enhanced by anaddition of any suitable electrolyte. A mass transport limited currentmay also be varied by manipulating the flow characteristics of theanolyte. When employing a dilute chlorite ion solution as an anolyte, itmay be economical to operate the process in a "single pass" mode,whereby the anolyte is not recycled or, alternatively, is recycled onlyas long as the level of impurities accumulated during the operation isacceptable.

Another parameter that requires optimization is temperature. Whilehigher temperatures generally enhance the gas transfer rates as well asreduce the cell voltage, they may also facilitate the rate of undesireddecomposition reactions involving either chlorine dioxide or chloriteions or a combination thereof. Higher temperatures generally alsoincrease the conductivity, thus lowering the cell voltage.

Yet another variable to be considered during the process optimization isthe flow characteristics of the anolyte/donating solution as compared tothe flow characteristics of the receiving solution. While it maygenerally be beneficial to increase the velocity of the anolyte/donatingsolution from the viewpoint of the mass transport limited currentdensity achievable as well as an enhancement of the gas transfer rate,there may be some limitations associated with gas membrane ability tosustain higher pressures resulting from higher velocities ("bubblepoint").

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to the drawing, concentrated sodium chlorite solution is fedby line 10 to a feed tank 12 wherein the sodium chlorite is mixed withrecirculated cell liquor to form a sodium chlorite feed stream which isfed by line 14 to an anode compartment 16 of an electrolytic cell 18.The cell 18 comprises a cathode compartment 20 separated from the anodecompartment 16 by a cation-exchange membrane 22.

In the anode compartment 16, the sodium chlorite undergoes electrolyticoxidation to form chlorine dioxide, while sodium ions are transferred,along with some water, from the anode compartment 16 to the cathodecompartment 20 through the cation-exchange membrane 22. An aqueouscatholyte is fed to the cathode compartment 20 by line 24, resulting inthe formation of an aqueous sodium hydroxide effluent stream in line 26.Alternatively, the sodium hydroxide stream may be recirculated throughthe cathodic compartment. During the recirculation, any desiredconcentration of sodium hydroxide may be achieved and maintained, forexample, by adding water, as required.

The aqueous solution of chlorine dioxide in the anolyte effluent isremoved from the anode compartment 16 and forwarded by line 28 to a gasmembrane unit 30. The gas membrane unit 30 contains a microporous,hydrophobic gas membrane 32 dividing the interior of the unit into twocompartments 34 and 36 and which permits gaseous vapors to transfertherethrough under suitable driving force. The chlorinedioxide-containing solution in line 28 is received in compartment 34 ofthe gas membrane unit 30 while water to be treated is fed by line 38 tothe other compartment 36.

In view of the difference in the partial pressure of chlorine dioxide onthe two sides of the gas membrane, chlorine dioxide is transferred fromthe chlorine dioxide-containing solution in compartment 34 to the waterin compartment 36 by gaseous phase transfer through the membrane 32 soas to treat the water, removed by line 40. While the invention isparticularly useful for the treatment by chlorine dioxide of water forconsumption, the invention includes the use of chlorine dioxide for thetreatment of any aqueous medium, including sewage and other waste watertreatments. In addition, the chlorine dioxide may be used in thetreatment of non-aqueous medium, for example, in tallow bleaching.Generally, the present invention may be employed in any disinfection,bleaching, sterilization or oxidation application involving chlorinedioxide.

The aqueous phase in compartment 34 preferably is maintained at a highertemperature or pressure than the water in compartment 36 to provide avapor pressure driving force, as a result of which water vapor istransferred along with the chlorine dioxide through the gas membrane 32from compartment 34 to compartment 36 (gas phase ClO₂, vapor phasedelivery).

The residual aqueous phase in compartment 34 containing unreactedchlorite and residual chlorine dioxide is recycled by line 42 to thefeed tank 12 for mixing with concentrated sodium chlorite feed in line10 and the process is repeated. The aqueous alkaline solution in line 26may be added to the treated water in line 40.

The drawing illustrates a continuous process of effecting watertreatment using pure chlorine dioxide, in which steps are taken toeffect water extraction from the chlorine dioxide generating process. Asmentioned above, using a concentrated (37 wt %) solution feed of sodiumchlorite, about 9 moles of water need to be removed for each mole ofchlorine dioxide produced.

This water removal is effected by transfer through the cation-exchangemembrane 22, generally in an amount of 2 to 5 moles, and by transferthrough the gas membrane 32, in an amount of about 4 to 7 moles, or moreor less, as required.

Alternatively, vacuum or partial vacuum can be applied to the receivingside in order to facilitate the transfer of chlorine dioxide gas andwater vapour. Such a system also may be employed to deliver gaseouschlorine dioxide, optionally in mixtures with water vapour and otherdiluting gases, such as nitrogen-oxygen or air, if required. A deliverysystem producing gaseous chlorine dioxide may have application not onlyto water treatment but also in other areas, such as in the bleaching ofpulp, where gaseous chlorine dioxide can be delivered directly to thetreated pulp without being previously dissolved in water.

While the two-step removal of water employed in the present inventionhas been described with respect to the generation of chlorine dioxideelectrolytically from sodium chlorite, the principles hereof areapplicable to other chlorine dioxide generating processes, for examples,those based on chlorate ion reduction, wherein the chlorate ionsoriginate from chlorate salts, chloric acid and mixtures thereof.Further, while the present invention has been described with respect tothe simultaneous transfer of chlorine dioxide and water vapor to arecipient medium, the principles hereof are applicable to the transferof other gases, such as chlorine monoxide, hypochlorous acid orchlorine, along with water vapor to a recipient medium. Gases, such ascarbon dioxide, oxygen or hydrogen, which may be formed during theoperation of the process of the invention can also be removed from thedonor medium to the recipient medium, if desired.

EXAMPLE

A plate and frame gas membrane module made using the elements of anElectro Syn cell (Electro Cell AB, Akersberga, Sweden) was evaluated.

The module was comprised of three PVDF frames of 1 cm thickness anddimensions L=53 cm, W=20 cm.

A 51 μm thick membrane made of "GORE-TEX" material (i.e. expandedpolytetrafluoroethylene, 40 psig minimum water entry pressure) supportedon PVC coated fiberglass scrim was inserted on each side of the centerframe. The resulting membrane exposed surface area was 0.08 m².

About 60 L of 10 wt. % NaClO₂ solution containing between 2 to 10 g/LClO₂ generated electrochemically was circulated through the modulemiddle compartment at different anolyte flow rates. Water, whichconstituted the recipient medium, was circulated through the first andthird frame in a co-current mode. The recipient medium was continuouslymonitored for any possible leak of ions from the anolyte through the gasmembrane. No measurable leak was observed in any of the experiments.

Samples of anolyte inlet and outlet solutions as well as water exitingthe module were analysed for ClO₂ content. The drop in ClO₂concentration in the anolyte due to transfer into the water compartmentswas evaluated using the log mean ClO₂ concentration difference,ΔClO_(2LM). ##EQU1## where: ClO_(2S1) =anolyte inlet ClO₂ conc.,(mol/cm³)

ClO_(2S2) =anolyte outlet ClO₂ conc., (mol/cm³)

ClO_(2W2) =water outlet ClO₂ conc., (mol/cm³)

From these calculations, the chlorine dioxide mass transfer coefficientwas calculated using the following equation: ##EQU2## where: V_(W)=water flow rate, (cm³ /min)

ClO_(2W2) =water outlet ClO₂ conc., (mol/cm³)

A_(t) =mass transfer area based on surface area of gas-liquid contact,(cm²)

In a parallel series of experiments, water vapor transfer from thesodium chlorite solution to the water compartments was evaluated. Thesystem consisted of the same module configuration and membrane materialas stated above. Water vapor transfer rates from chlorite solutions atdifferent concentrations and temperatures were evaluated by determiningthe chlorite solution water loss at different time intervals forspecific chlorite solutions/water temperature log mean differences,calculated as: ##EQU3## where: T_(S1) =chlorite sol. inlet temperature,(° C.)

T_(S2) =chlorite sol. outlet temperature, (° C.)

T_(W1) =water inlet temperature, (° C.)

T_(W2) =water outlet temperature, (° C.)

Example 1

In a first series of experiments, the ClO₂ mass transfer rate throughthe membrane as a function of anolyte flow was evaluated for a 10 wt %NaClO₂ donating solution (i.e. anolyte) and H₂ O as the receivingsolution. The variation in K values with increasing anolyte flow may beseen from the results listed in the following Table 1 below:

                                      TABLE 1    __________________________________________________________________________       Water           Anolyte               Anolyte                     Water Temp.    Run       Flow           Flow               Temp. (° C.)                     (° C.)                           ClO.sub.2 conc. (g/L)                                    ΔClO.sub.2LM                                          K.sub.ClO2                                                ClO.sub.2 Flux*    No.       (L/min)           (L/min)               IN OUT                     IN OUT                           S1 S2 W2 (mol/cm.sup.3)                                          (cm/s)                                                (g/m.sup.2 min)    __________________________________________________________________________    1  2   2   23.6                  21.7                     5.4                        8.3                           4.263                              4.234                                 0.047                                    6.26 × 10.sup.-5                                          4.68 × 10.sup.-4                                                1.1    2  2   2   23.2                  21.4                     5.4                        8.2                           4.266                              4.201                                 0.045                                    6.24 × 10.sup.-5                                          4.44 × 10.sup.-4                                                1.1    3  2   5   23.6                  22.6                     3.5                        7.1                           4.107                              4.085                                 0.066                                    6.02 × 10.sup.-5                                          6.82 × 10.sup.-4                                                1.6    4  2   5   23.1                  22.1                     3.6                        7.0                           4.088                              4.085                                 0.065                                    6.01 × 10.sup.-5                                          6.66 × 10.sup.-4                                                1.6    5  2   8.7 21.4                  20.8                     4.1                        7.3                           4.140                              4.120                                 0.078                                    6.06 × 10.sup.-5                                          7.96 × 10.sup.-4                                                1.9    6  2   8.7 20.8                  20.2                     3.7                        6.9                           4.091                              4.088                                 0.077                                    6.01 × 10.sup.-5                                          7.88 × 10.sup.-4                                                1.9    7  2   16.0               24.5                  24.0                     3.3                        8.2                           2.445                              2.373                                 0.064                                    3.52 × 10.sup.-5                                          1.12 × 10.sup.-3                                                2.7    8  2   16.0               23.2                  22.7                     3.5                        7.8                           2.422                              2.396                                 0.062                                    3.53 × 10.sup.-5                                          1.09 × 10.sup.-3                                                2.6    __________________________________________________________________________     *Calculated ClO.sub.2 flux rates normalised for S.sub.1 = 4.0 g/L     ClO.sub.2

Example 2

In a parallel series of experiments, water vapor transfer rates throughthe membrane material were evaluated using different NaClO₂ solutionconcentrations and ΔT_(LM) values between the donating and receivingsolution, (i.e. H₂ O).

The results for a 10 wt. % NaClO₂ solution are listed in the followingTable 2:

                  TABLE 2    ______________________________________             Time   ΔT.sub.LM                             Water Loss                                     Water Flux    Run No.  (min)  (° C.)                             (cm.sup.3)                                     (g H.sub.2 O/m.sup.2 min)    ______________________________________    1        90     9.0      58      8    2        60     9.0      40      8    3        30     15.4     61      25    4        30     15.4     62      26    5        30     20.4     92      38    6        30     20.4     93      39    7        30     24.1     119     50    8        30     24.1     115     48    ______________________________________

As can be seen from the comparison of typical fluxes of water andchlorine dioxide expressed in moles/m² min contained in Tables 1 and 2,the ratio of such fluxes is typically significantly higher than 10:1,thus allowing removal of more water that would have been, otherwise,required when using 37% sodium chlorite feed solution. Hence, lessconcentrated feed solution may be employed, if desired, without anynegative effects on the overall water balance in the process.

Example 3

An electrochemical cell equipped with a graphite or DSA-O₂ ® anode,nickel cathode and Nafion 417 cation-exchange membrane was used todetermine the water transfer rates at various concentrations of theanolyte. The number of moles of water transferred per each mole ofsodium ions varied from about 3 to 4 for a concentrated anolyte (15 wt.%) to 4 and over for a 10 wt. % or less concentrated anolyte.

The use of a less concentrated anolyte (5 to 10 wt. %) resulted in avery high chemical efficiency in the range of 99 to 100%. The gaseousproduct contained at least 99.5 wt. % chlorine dioxide and less than 0.5wt. % chlorine. No measurable quantities of chlorate ions were formed.The current efficiency values ranged between 95% and 99% dependent oncurrent density which was varied between 1 and 4 KA/m². At highercurrent densities and for a given flow characteristics and chloriteconcentration, the contribution from the oxygen evolution reaction wasgenerally higher resulting in a decrease in current efficiency.

In experiments involving sodium hydroxide as a catholyte thebackmigration effect of hydroxyl ions was determined based on the netproduction of sodium hydroxide in the cathodic compartment. The loss incathodic current efficiency, which can be attributed to the hydroxylions backmigration through the cation exchange membrane, was found to bein the range of 3 to 5% for 0.1 to 0.2 molar NaOH.

The addition of a concentrated phosphate buffer to the anolyte was foundto be a very effective method of minimization or elimination of thechlorate formation effect within the alkaline boundary layer believed tobe formed on the side of the cation exchange membrane facing the anode.

The use of a bicarbonate/cabonate mixture as a catholyte resulted in anearly complete elimination of the backmigration effect from the cathodecompartment.

SUMMARY OF THE DISCLOSURE

In summary of this disclosure, the present invention provides a novelprocedure for forming chlorine dioxide and utilizing the chlorinedioxide in a beneficial manner. Modifications are possible within thescope of this invention.

What we claim is:
 1. A method of removing at least one dissolved gas andwater from an aqueous solution of the at least one gas, whichcomprises:contacting said aqueous solution with one face of hydrophobicmicroporous membrane, and providing a differential of partial pressureof both said at least one gas and water vapor between said aqueoussolution and a recipient medium in contact with the opposite face ofsaid hydrophobic microporous membrane, whereby both the at least one gasin gaseous form and water vapor pass through the membrane.
 2. The methodof claim 1 wherein said at least one dissolved gas is chlorine dioxide.3. The method of claim 2 wherein said chlorine dioxide is formed byelectrochemical generation from an aqueous solution of an alkali metalor alkaline earth metal chlorite.
 4. The method of claim 3 wherein saidalkali metal chlorite is sodium chlorite.
 5. The method of claim 4wherein said sodium chlorite solution is substantially free fromimpurities.
 6. The method of claim 4 wherein said electrochemicalgeneration is effected by feeding said aqueous solution of sodiumchlorite to an anodic compartment of an electrochemical cell divided byat least one ion-permeable separator from a cathode compartment to whichan aqueous electrolyte is fed.
 7. The method of claim 6 wherein saidaqueous solution of sodium chlorite present in said anodic compartmenthas substantially neutral pH.
 8. The method of claim 6 wherein saidelectrochemical cell is a two-compartment cell and said ion-permeableseparator is a cation-exchange membrane.
 9. The method of claim 6wherein said electrochemical cell is a three-compartment cell and saidat least one ion-permeable separator comprises two cation-exchangemembranes.
 10. The method of claim 6 wherein said electrochemical cellis a three-compartment cell and said at least one ion-permeableseparator comprises an anion-exchange membrane separating the anodecompartment and a central compartment and a cation-exchange membraneseparating the central compartment and the cathode compartment and saidaqueous sodium chlorite solution is fed to said central compartment. 11.The method of claim 6 wherein backmigration of hydroxyl ions from saidcathode compartment to said anode compartment occurs through saidcation-exchange membrane and hydrogen ions are generated in said anodecompartment to effect at least partial neutralization of saidbackmigrating hydroxyl ions as well as any hydroxyl ions introduced intosaid anolyte compartment with said sodium chlorite feed.
 12. The methodof claim 11 wherein said hydrogen ions are formed by electrochemicaldecomposition of water at the anode in accordance with the equation:

    2H.sub.2 O→O.sub.2 +4H.sup.+ +4e

and the co-produced oxygen is vented from the anode compartment.
 13. Themethod of claim 6 wherein a buffer is present in said anode compartmentto effect neutralization of said backmigrating hydroxyl ions as well asany hydroxyl ions introduced into said anolyte compartment with saidsodium chlorite feed and hydrogen ions produced in said anodecompartment.
 14. The method of claim 6 wherein sodium ions aretransferred through the cation-exchange membrane into the cathodecompartment and form sodium hydroxide therein, which is recovered fromthe cathode compartment.
 15. The method of claim 6 which is carried outcontinuously by effecting said removal of chlorine dioxide fromelectrolyzed aqueous sodium chlorite solution, recycling electrolyzedsodium chlorite solution following said chlorine dioxide removal to saidanodic compartment and feeding make-up sodium chlorite to said recycledsodium chlorite solution, and wherein water is removed from saidelectrolyzed aqueous sodium chlorite solution to maintain asubstantially uniform concentration of aqueous sodium chlorite solutionentering said anode compartment.
 16. The method of claim 15 wherein saidelectrolyzed aqueous sodium chlorite having chlorine dioxide dissolvedtherein is passed from said anode compartment to a gaseous transfer zonecomprising said hydrophobic microporous membrane dividing the zone intoa first chamber through which said electrolyte aqueous sodium chloritesolution is passed, and a second chamber through which said chlorinedioxide recipient medium is passed, and said chlorine dioxide and watervapor are passed through said hydrophobic microporous membrane to saidrecipient medium.
 17. The method of claim 16 wherein said electrolyzedaqueous sodium chlorite solution is hotter by from about 0.1° to about100° C. than said recipient medium to facilitate said water vaporpassage, to provide said partial pressure differential with respect towater vapor.
 18. The method of claim 17 wherein said electrolyzedaqueous sodium chlorite solution has a temperature of about 5 to about100° C. and said recipient medium has a temperature of about 1° to about80° C.
 19. The method of claim 16 wherein the recipient medium is waterrequiring treatment by the passed chlorine dioxide.
 20. The method ofclaim 16 wherein said hydrophobic microporous membrane is constructed ofexpanded polytetrafluoroethylene.
 21. The method of claim 6 whereinsodium hydroxide produced in the cathode compartment is added to saidrecipient medium before or after receipt of chlorine dioxide therein.22. The method of claim 6 wherein said aqueous electrolyte fed to thecathode compartment contains bicarbonate or carbonate ions or a mixturethereof and the product of cathodic reaction is enriched in carbonateions.